UBIFS: reserve more space for index

At the moment UBIFS reserves twice old index size space for the
index. But this is not enough in some cases, because if the indexing
node are very fragmented and there are many small gaps, while the
dirty index has big znodes - in-the-gaps method would fail.

Thus, reserve trise as more, in which case we are guaranteed that
we can commit in any case.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>

fs/ubifs/find.c

@@ -290,9 +290,14 @@ int ubifs_find_dirty_leb(struct ubifs_info *c, struct ubifs_lprops *ret_lp,
 		idx_lp = idx_heap->arr[0];
 		sum = idx_lp->free + idx_lp->dirty;
 		/*
-		 * Since we reserve twice as more space for the index than it
+		 * Since we reserve trice as more space for the index than it
 		 * actually takes, it does not make sense to pick indexing LEBs
-		 * with less than half LEB of dirty space.
+		 * with less than, say, half LEB of dirty space. May be half is
+		 * not the optimal boundary - this should be tested and
+		 * checked. This boundary should determine how much we use
+		 * in-the-gaps to consolidate the index comparing to how much
+		 * we use garbage collector to consolidate it. The "half"
+		 * criteria just feels to be fine.
 		 */
 		if (sum < min_space || sum < c->half_leb_size)
 			idx_lp = NULL;